1. Technical Field
The present invention is related generally to superconducting materials. More particularly, the present invention is related to new, high temperature superconducting ceramic compositions and a process for synthesizing the same.
2. State of the Art
Most ceramic materials in commercial applications are currently prepared by pressing together oxide powders and sintering or firing the pressed body into a strong, dense article. For such ceramic materials, the best mechanical properties such as tensile and compressive strength and fracture toughness are achieved if the final sintered body has a morphology consisting of fine oxide grains which are completely sintered together to form an object which has the minimum possible porosity (highest theoretical density). However, in most instances, the final grain size and degree of sintering in the ceramic are controIled by the properties of the powders used in the pressed body. The grain size of the ceramic is largely controlled by the size and degree of agglomeration of the starting powder particles. The larger the particle size, the larger are the grains in the final ceramic. In general, the pore size and degree of sintering are controlled by the particle size. Large powder particles are less active in sintering, producing more porous materials. The larger the porosity in the material, the poorer are the mechanical properties. Therefore, to form dense, fine-grained ceramics, it is desirable to start with fine particles in the powder. Control of particle shapes can also be important. Ideally, particles in the starting powder should be smaller than about 1 micron in size.
Many ceramic materials contain several different oxide constituents. For these materials, desired ceramic phases are produced by solid state reactions between constituent oxides during high temperature sintering. If the individual oxide particles are large, and if the solid state reactions are sluggish, it can be difficult to drive the desired reactions to completion, resulting in the production of ceramic materials which are compositionally inhomogeneous. Compositional inhomogenieties can produce unwanted phases and chemical gradients within the material which can degrade desired properties. Problems of inhomogenieties can best be solved by dispersing the individual components on as fine a scale as possible to minimize diffusion distances. If individual oxides are used, it is advantageous to use particles which are as fine as possible to promote reactions between oxides. However, greater homogenieties can be achieved if each powder particle contains a homogeneous mixture of the desired components before it is pressed into the pellet.
The new high critical temperature superconducting ceramics such as YBa.sub.2 Cu.sub.3 O.sub.7-x (1,2,3) provide an example of how mixed-oxide processing can degrade ceramic microstructures and properties. Superconducting ceramics are conventionally prepared by the mixed-oxide route. For 1,2,3 a stoichiometric mixture of Y.sub.2 O.sub.3, BaCO.sub.3, and CuO powders are ball-milled to break up powder agglomerates and to mix the individual components. The ball-milled powders typically contain particles and agglomerates of individual components which are tens of microns in diameter. To make the 1,2,3 material, the ball-milled powders are calcined to thermally decompose the BaCO.sub.3 and to drive the solid state reactions between the individual oxides. Since the large grained powders are not particularly reactive, calcining temperatures of over 900.degree. C. are required to make the 1,2,3 material. The high calcining temperature promotes further grain growth and partial sintering between particles. When the calcined powders are pressed into pellets and sintered, the resulting ceramics typically have low densities (&lt;80% theoretical density), contain individual grains which can be larger than 50 microns in size, and can be so porous that visible light can pass through the entire sample. Because of their low densities, the ceramics have poor structural integrity and often crumble to the touch. Because of incomplete reactions during sintering, the pellets often contain extraneous phases, or have the desired phases with a range of oxygen stoichiometries. Therefore, the resulting material often has a low volume fraction of superconducting material. To prepare good superconductors, it is necessary to regrind the ceramic into a powder, resinter into ceramic parts, and recycle back and forth between regrinding and sintering until good properties are achieved. Clearly, the regrinding operation is highly undesirable, since it adds unnecessary processing steps and can lead to sample contamination. Heretofore, the only way to use mixedoxides to prepare dense superconducting material was to use hot isostatic or hot uniaxial pressing techniques in which high pressures are applied to the body at high temperatures to force sintering to occur. However, hot-pressing is limited in terms of sample size, sample geometry, and sintering atmosphere, making it of limited utility in the fabrication of useful superconducting parts.
Because of the problems inherent in processing mixed-oxides, alternate preparation methods have been studied, including rapid solidification (splat-cooling) from melts of superconducting phases, oxidation of metal alloys, chemical vapor deposition and molecular beam epitaxy techniques. An important subset of ceramic processing techniques are the so-called "chem-prep" methods which involve dissolving the metals which comprise the oxide in either aqueous or organic solutions to promote mixing of the different elemental constituents. The metals can be removed from solution using a variety of methods, the simplest of which is evaporation (used in techniques such as spray drying or freeze drying). A related technique is the Pechini method, in which metals are dissolved with citric acid in ethylene glycol. The solution is thermally treated to form a gel which is then further heat treated to produce desired oxide phases.
However, a process which gives fine-grained, chemically homogeneous powders for producing superconductive ceramic materials has not heretofore been known or described.